1. Field of the Invention
This invention relates to a method and apparatus for providing enhanced sensitivity in detecting atomic and molecular species of interest through their absorption spectra. For laboratory measurements, spectroscopic studies, White-cell in-situ monitoring and long-path stratospheric monitoring, the species being measured are gases at low pressures. For example, it has been determined that White cells of base pathlength in the range 1.0-4.0 m may be particularly susceptible to unwanted interference fringe generation which reduces the sensitivities attainable using tunable laser absorption techniques. These fringes are caused by multiple reflections of light between any two or more surfaces located within the light-source-to-detector path.
In many applications of tunable diode laser (TDL) spectroscopy to molecular detection, it is not possible to "remove" the molecular gas being measured. The fringes may therefore not be independently recorded and the molecular contribution extracted by subtraction techniques. Any post-detection data-handling technique must use one identity to separate the two contributions.
In accordance with this invention, a Brewster-plate spoiler is oscillated within the passive cavity which is formed outside of the laser resonator's cavity. This passive cavity is formed by the surfaces from which the multiple reflections, and therefore fringe generation occurs. Frequency-related interference effects in a tunable laser absorption spectrum are minimized by this invention.
2. Brief Description of the Prior Art
Tunable diode laser spectrometers are employed for high sensitivity studies. Laboratory and field studies include multipass cell measurements and/or long pathlength atmospheric measurements using retroreflectors. Such devices have been limited in precision measurement sensitivity by the presence of optical interference fringes. When used to measure molecular absorptions smaller than 0.01%, even though optical beam noise is reduced by increased integration time, actual measurement sensitivity is limited by the presence of interference fringes.
The sensitivity of laser absorption measurements is limited by the production of Fabry-Perot interference fringes. Such fringes are generated through multiple beam interferences between optical surfaces located within the source-to-detector path of the spectrometer system. The fringes may result from laser transmission through individual optical elements, such as windows or lenses, or through air and vacuum paths separated by the surfaces of different system elements. Even if very careful optical alignment procedures are practiced, such efforts cannot completely remove interference fringes.
Fringes of peak-to-peak optical depth of a few parts in 10.sup.4 for example, are produced by surface reflectivities as low as 0.01%. For multipass absorption cells, the problem is more serious due to the increased beam overlap. Multipass absorption attempts to maximize the effective pathlength by increasing the number of optical passes. Thus, multipass systems have increased interference.
Tunable diode laser (TDL) spectrometers are particularly suited to high sensitivity studies, in part because they may be readily frequency-modulated. Harmonic techniques are used to measure molecular absorptions smaller than 0.01%. At this low molecular absorption level, optical beam noise may be reduced by increased integration time. Notwithstanding enhanced tuning of the laser and reduction in optical beam noise, the measurement sensitivity is, nevertheless, limited by the presence of the above-noted optical interference fringes.
Various attempts to improve sensitivity have been made. For example, Reid et al. successfully used a jitter-modulation technique that is described in Applied Optics, 19 (1980) p. 3349, in an article entitled "Sensitivity Limits of a Turnable Diode Laser Spectrometer, with Application to the Detection of NO.sub.2 at the 100-ppt Level." Reid et al. later developed a two-tone modulation technique disclosed in "Harmonic Detection with Turnable Diode Lasers: Two-Tone Modulation," to reduce optical fringing the latter being published in Applied Physics B, 29, (1983) p. 279. A related approach described in "Improvement of Etalon-fringe Immunity in Diode-Laser Derivative Spectroscopy," was taken by Koga et al as reported in Memoirs of the School of Engineering, Okayama University, 16, (1981) p. 21. Koga et al. proposed modulating the TDL current with an exponential function approximating an inverse integrated raised cosine profile.
All three of the referenced techniques attack the fringing problem through laser modulation. They use complex modulations which produce harmonic signal sizes and nonconventional linewidths. The complex modulation of this prior art approach makes transformation back to gas concentration measurements unnecessarily difficult. The above-noted prior art is relevant to a general concept of improving a laser's performance. It has limited relevance to this invention.
Tuning a Fabry-Perot cavity using an internal Brewster plate has been previously applied to cw dye lasers as described in "Direct Optical Measurement of Sodium Hyperfine Structure using cw Dye Lasers and an Atomic Beam," by Schuda et al Applied Physics Letters, 22, (1973) p. 360. In accordance with this invention, an exterior cavity resonance is defined. Rather than tuning by use of a Brewster plate, as suggested by the above-noted prior art, the exterior cavity is spoiled, in accordance with this invention, by oscillating a plate about an axis 90.degree. to the plane of the laser beam's incidence.
The following patents were deemed relevant to a prior art search on the disclosed invention: U.S. Pat. No. 3,731,224 to Dienes et al.; U.S. Pat. No. 4,268,800 to Johnson, Jr. et al.; U.S. Pat. No. 3,435,371 to A. D. White; U.S. Pat. No. 3,868,592 to Yarborough et al.; U.S. Pat. No. 4,233,569 to Liu; and U.S. Pat. No. 4,438,517 to Bobb et al.
Dienes et al. U.S. Pat. No. 3,731,224 discloses a compensated folded resonator having a Brewster element 21 within the laser cavity for introducing an astigmatic effect which cancels an already existing one due to the resonator mirrors, elements 18 and 19. See Column 3, lines 20-40. The Brewster plate solves a different problem by a different structure than that of the present invention. Furthermore, the folding of spherical mirrors in Dienes et al. U.S. Pat. No. 3,731,224 inherently creates an astigmatic effect within the laser cavity itself. By cancelling that astigmatism, instability in the resonance frequency and reduced laser output power can be overcome. Dienes et al. U.S. Pat. No. 3,731,224 does not have an oscillating plate nor does it correct for interference fringes outside of the laser cavity.
Johnston, Jr. et al. U.S. Pat. No. 4,268,800 discloses, in FIG. 1A, a known prior art use of tipping an inserted glass plate about a small range of angles near Brewster's angle to change the length of optical path in a dye laser cavity to produce a frequency scan. The Brewster plate 30 is mounted within the laser cavity near the vertex of the incident beam and the reflected beam. The Brewster plate 30 is tipped or scanned through a small angle, thereby cancelling the lateral displacement of the incident and reflected beams. See Column 2, lines 20-35. Brewster plate 30 is tipped to selectively tune the laser to a predetermined output frequency. See Column 1, lines 43-67. It should be noted that Johnston, Jr. et al. U.S. Pat. No. 4,268,800 (Column 1, lines 26-29) also lists as background prior art one of the literature references cited above.
U.S. Pat. Nos. 3,435,371, 3,868,592, 4,233,569 and 4,438,517 disclose, as did Johnston, Jr. et al. U.S. Pat. No. 4,268,800, a technique for tuning a laser. In particular, U.S. Pat. No. 3,868,592 uses a birefringent plate 10 disposed at the Brewster angle for tuning the laser. See Column 3, lines 31-47.